This article considers porous titanium, with a focus on its properties and applications.
Image Credit: Alexey Rezvykh/Shutterstock.com
The growth of many industries in the early 1960s, particularly aviation and astronautics, required the development of novel materials. During this time, titanium and titanium alloys also gained attention due to their excellent properties. Since 1910, porous titanium, manufactured using the Hunter technique, has been recognized as a titanium sponge, but it received significant industrial consideration only after the Kroll technique was developed for the industrial manufacturing of titanium sponges.
Commercially used pure titanium and titanium alloys have hexagonal lattice structures, comprise soluble substitutional elements (Al or Sn), and may or may not contain interstitial components (such as O, N, or C).
Properties of Porous Titanium
Porous materials contain empty spaces with varying pore size distributions and pore morphologies. Porous titanium structures with a porosity of up to 98% are feasible (using hollow titanium spheres), and the percentage of pores and the mean pore size vary according to the application. Porous titanium structures having uniform size and shape distribution are classified into three types based upon structure - bimodal structure, gradient structure, and honeycomb structure.
Compared to solid materials, porous structures display distinct operational and mechanical qualities. Relative density (RD) values for porous structures are in the range of 0.003- 0.3. However, when porous titanium is employed in medicine, relative density can range between 0.3 and 0.85. Microporosity is critical in medicine because it increases the implant surface energy during osseointegration.
The Young's modulus of porous materials is dependent on the types of pores in porous materials and stress conditions. Due to the smaller cross-section and stress localization in thin walls of porous materials, the strength of porous materials is significantly reduced compared to solid materials. As a result, compression mode load-bearing structures are primarily utilized in their applications of porous materials such as porous titanium. The compressive strength of porous pure titanium (porosity 50%–70%) may reach 25–5 MPa.
Three phases can be used to characterize the fatigue behavior of porous materials. A gradual change in strain characterizes the first stage, while the second involves the accumulation of the smallest amount of strains, and the last stage results in sample failure after a restricted number of cycles, which is preceded by a fast strain rise. Various studies have observed that the fatigue performance of porous titanium is higher than aluminum and nickel.
Further Reading on AZoM: The Use of Graphene in Dentistry
Permeability indicates a material's ability to transport fluids/gases in the presence of a pressure differential. Darcy's law is used to estimate the permeability of porous materials and the permeability of porous titanium is mainly determined by its fluid and porosity structure.
Titanium and its alloys and other metals exhibit varying degrees of corrosion resistance depending on their chemical structure. Porous titanium shows higher corrosion resistance compared to its solid counterparts, mainly due to the effect of sinters surface oxidation.
Applications of Porous Titanium
Porous titanium is used in aerospace, medical, automotive, space industry, and military applications. In the medical industry, properties of porous titanium like biocompatibility and excellent corrosion resistance are very significant. Commercially available dental implants and bone implants are usually made of solid titanium or porous titanium.
The main reasons for using porous titanium in the aerospace industry are weight reduction, maximum service temperature, excellent corrosion resistance, and the ability to absorb energy.
The lightweight construction of porous metals, their ability to reduce the amount of vibration control required for reciprocating weight, and the lower rotational mass than steel are significant for automobile industry applications. Flue cells, flow systems (fillers, catalytic converters), and electrochemical applications also use porous titanium.
Pure state titanium has inferior mechanical properties but higher corrosion resistance compared to titanium alloys. Numerous studies are now being conducted to devise ways to improve the mechanical characteristics and corrosion resistance of titanium alloys. The majority of these studies are focused on addressing chemical and phase composition optimization.
Recent studies on porous titanium are also focused on its application in the biomedical (implants), automobile, and space sectors. In one of the recent research, porous titanium was used to manufacture porous cranial implants to overcome the limitations of traditional implants. In addition to this, porous titanium has also been integrated with 3D printing technology to improve its utility in the biomedical, aviation, and space sectors.
A fundamental constraint in using porous materials is their cost, which is determined by the raw material and production expenses. Additionally, controlling the morphology of porosity is difficult and subject to unpredictability. Certain alloying elements are not permitted in porous titanium alloys used for medical purposes because of their toxicity or potential for causing severe adverse effects on the human body.
References and Further Reading
H.P. Tang, J. Wang, Ma Qian, 28 - Porous titanium structures and applications, Editor(s): Ma Qian, Francis H. (Sam) Froes, Titanium Powder Metallurgy, Butterworth-Heinemann, 2015, Pages 533-554, ISBN 9780128000540, https://doi.org/10.1016/B978-0-12-800054-0.00028-9.
K. Pałka, R. Pokrowiecki, M. Krzywicka, Chapter 3 - Porous titanium materials and applications, Editor(s): Francis Froes, Ma Qian, Mitsuo Niinomi, Titanium for Consumer Applications, Elsevier, 2019, Pages 27-75, ISBN 9780128158203,https://doi.org/10.1016/B978-0-12-815820-3.00013-7